Power electronics for wind turbines References ECE 566: Grid Integration of Wind Energy Systems S. Suryanarayanan Associate Professor ECE Dept. Suryanarayanan ECE 566 Lecture/Week 7
Power electronics for wind turbines References Reminders and notifications Mid-term exam scheduled for October 14, 2014 Next 1 Tuesday 75-minutes take-home exam 645–8 PM (Mtn time) Closed notes, closed book, etc. 1 page cheat sheet allowed: letter-sized 2-sided; to be scanned and submitted along with answers Access and submission via RamCT Blackboard Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Capacitor bank controller [1] Capacitor bank Cap banks are used with induction generators to supply reactive power (VARs) Traditionally, mechanically-switched cap banks were used with Type-I and Type-II wind turbines to minimize electrical power drawn by the induction machine from the grid Advantages of mechanically-switched banks: easy to control and cheap Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Capacitor bank controller [1] Capacitor bank Induction generators in wind turbines can have a full load dynamic compensation A certain number of caps may be connected or disconnected continuously based on average reactive power demand over a fixed window of time Since this reactive power drawn is dependent on wind speeds, it may lead to an excessive number of switching leading to: Transient over-voltages Mechanical and electrical stress of capacitors Number of switching operations will affect the capacitor lifetime, and also the wind turbine and gearbox reliability Over-voltage on the grid-side may cause heavy loading and damages to the wind turbine leading to increased maintenance costs Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Capacitor bank controller Capacitor bank control figure taken from [2] Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Capacitor bank controller Reactive power control in wind farm [3] Local capacitor bank control at terminals of WTG For Type-I WTG, 2 sets of cap bank configurations, each 1 corresponding to one set of poles, is included Based on the number of poles in the operation, the banks 2 are switched appropriately Cable capacitance provides reactive compensation Substation power factor capacitors STATCOM or SVC Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Capacitor bank controller Capacitor bank: Hybrid reactive compensator While mechanical switching is easy when the demand is known, it is difficult to do so when the demand for reactive power is variable such as in the wind turbine Local capacitor banks can be used with power electronic solutions such as a static compensator (STATCOM) to provide a hybrid compensator [4] Advantages Dynamic control of power factor of wind farm 1 Independent control of reactive power at the turbine 2 terminals, thus reducing losses in the plant Available resources can be used for optimal reactive power 3 management Smooth electronic control instead of multiple mechanical 4 switching events Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [1] What is it? A cheap and efficient power electronic component used for 1 the interconnecting Types I and II WTG to the grid At start-up, induction generators produce high currents 2 (inrush), approximately 7–8 times the rated value which decays with time as the rotor picks up speed If left unaddressed, this momentary condition may damage 3 grid components and/or cause severe voltage disturbances Protection devices must not trip for this momentary 4 situation Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [1] What is it? A soft starter has two thyristors connected in anti-parallel 1 for each phase By controlling the firing angle ( α ) of the thyristors, a 2 smooth interconnection of the Types I and II WTG to the grid can be achieved Once the inrush currents subside and steady rated current 3 flows, the thyristors are by-passed to allow direct-coupling to the grid Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [6], [5] Firing a thyristor A thyristor is a 3-junction 4-layered semiconductor device 1 with alternating P and N types material Also known as silicon controlled rectifiers (SCR) 2 Three terminals known as anode, cathode, and gate 3 “A thyristor can be switched off if the external circuit 4 causes the anode to become negatively biased (a method known as natural—or line—commutation). In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.” [5] Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter Firing a thyristor. Figure taken from [5] Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [6], [5] Firing a thyristor Unlike diodes, the SCR does not automatically start 1 conducting when a positive voltage across the anode-cathode is applied Rather, the conducting state is achieved by supplying the 2 gate terminal with an impulse current; this is knows as ‘firing’ a thyristor Once fired, the thyristor behaves like a diode until an 3 off-state is reached Off-state is reached by letting the conducting current value 4 fall below a minimum value (holding current) Transition from off-state to conducting state is achieved by 5 firing new pulse current sequence to the gate Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [6], [5] Firing a thyristor Unlike diodes, the SCR does not automatically start 1 conducting when a positive voltage across the anode-cathode is applied Rather, the conducting state is achieved by supplying the 2 gate terminal with an impulse current; this is knows as ‘firing’ a thyristor The point on the waveform at which the thyristor is 3 triggered into conduction is called the firing angle ( α ) Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [6], [5] Firing a thyristor. Figure from [7] Once fired, the thyristor behaves like a diode until an 1 off-state is reached Off-state is reached by letting the conducting current value 2 fall below a minimum value (holding current) Transition from off-state to conducting state is achieved by 3 firing new pulse current sequence to the gate Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [6], [5] Firing a thyristor. Figure from [7] Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [7] Firing a thyristor Types of firing Zero-voltage cross-over: The thyristor is turned on only 1 when the instantaneous sinusoidal voltage at the gate reaches zero value Phase-angle control: By varying the phase angle, the timing 2 of the triggering of gate pulses is controlled (via a delay) Suryanarayanan ECE 566 Lecture/Week 7
Capacitor bank controller Power electronics for wind turbines Soft starter References Converters Future of power electronics in wind farms Soft starter [8] Soft starter A soft starter has 6 thyristors—2 per phase—connected in anti-parallel or back to back configuration To limit the rate of change of voltage across the thyristors, a snubber (RC) circuit is used Suryanarayanan ECE 566 Lecture/Week 7
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